The present invention relates to a system for determining shaft load parameters in a rotating shaft and, more particularly, to a system for non-contact determination of shaft windup angle per unit length, torque output and power output in a rotating shaft.
There are many prior art systems for detecting torque output and power output in a rotating shaft such as a shaft in a turbine or other such rotating equipment. Some prior art systems use axially spaced proximity probes which detect the passing of shaft keyways or targets which are placed on the shaft. However, with such prior art systems, the probe measurements may be inaccurate due to the effects of vibrations or position changes by either the shaft, the probes, or both which can affect target arrival times and produce an inaccurate shaft windup angle measurement.
One prior art torque measuring system described in U.S. Pat. No. 4,995,257 measures transient shaft torque using two separately mounted stationary probes to sense the passing of two targets or marks longitudinally spaced apart on a rotating shaft. The prior art system detects changes in the target passing times to discern transient shaft torques. For the system to yield accurate measurements, particularly for non-transient torques, the probes must be absolutely stationary with respect to the shaft. If the probes vibrate or displace relative to the shaft or to each other, then the resulting inaccuracies may significantly and adversely effect the torque measurements. Since it is almost impossible to maintain the probes absolutely stationary with respect to the shaft, the resulting measurements taken by the prior art system tend to be inaccurate.
Rotating shaft torque measurement is particularly useful in power generation facilities. Most generators are driven by a continuous train of several turbines. A continuous train of turbines comprises a plurality of turbines with in-line shafts each contributing torque, the sum of their torques driving the generator at a required speed. Sometimes, there is a condition of reduced torque in one of the various turbines in the turbine train due, for example, to labyrinth seal wear, blade erosion, steam chest valving problems, etc. A turbine having a lagging output torque must be compensated for by the other turbines in the train to keep the generator output constant. A lagging turbine effects the overall efficiency of the train, but may not be discernible because of the fact that the other turbines compensate for the lagging turbine. Proper shaft torque measurement permits the identification of a single turbine having a lagging output torque so that the problem can be noted and corrected before further deterioration of that turbine or the turbine train occurs.
The present invention is directed to a system for accurately and consistently determining shaft load parameters including shaft windup angle per unit length, torque output and power output in a rotating shaft. The shaft windup angle per unit shaft length is defined as the torque induced windup angle of the shaft over a given length of the shaft. The system preferably comprises four targets which are affixed to the rotating shaft and four probes which are commonly supported on a frame which can move and vibrate as a rigid body. The probes are fixedly positioned on the rigid frame around the shaft at spaced axial locations for sensing the passing of the targets as the shaft rotates. The probes being mounted on the rigid frame and the frame being supported so as to move essentially as a rigid body allows for an accurate shaft windup angle measurement to be obtained which is not effected by shaft or frame vibration and/or displacement. Accuracy and resolution are improved by averaging over multiple revolutions of the shaft. The windup angle resolution of the present invention approximates one millionth of a degree thereby making it possible to discern as little as a three foot-pound torque on a 10 inch diameter steel shaft with an axial probe spacing of only 3 inches. The present invention is also useful in determining the torque or power being supplied to a pump in a turbine driven pump application, and for measuring absolute and relative propeller shaft powers for multi-screw ships.